Optical discs are commonly used for data storage. That is, data is written to and read from such discs, by means of an optical source.
In order to maximize the data storage capacity of an optical disc, data is written to the disc with a fixed data density. That is, the spatial separation of the data points on the disc is constant. In order to allow a constant data rate, while data is being written to or read from the disc, the disc is rotated with a constant linear velocity (CLV). That is, the angular velocity varies, as the optical source moves from one part of the optical disc to another. Specifically, the disc rotates with a higher angular speed when data is being written to or read from an inner part of the disc, and with a lower angular speed when data is being written to or read from the outer part of the disc.
Different optical disc formats are known, and, in some formats, a disc is provided with one or more defect management areas, which may for example be located in the radially outer part of the disc. One such format is defined in the Mount Rainier specification, which is described for example in the document “Mt Rainier An Explanation” published by Koninklijke Philips Electronics NV, and available on the internet in November/December 2002 at www.mt-rainier.org. The defect management area may be used when writing data to the disc. If an error occurs while writing data to the disc, for example because of a defect in the disc, the data is written to the defect management area of the disc instead.
However, as mentioned above, in order to maintain the same data rate (that is, the same number of bits of data which are written to the disc each second), while writing to the outer area of the disc, it is necessary to decrease the rotational speed of the disc. Similarly, when writing to the defect management area has ended, it is again necessary to change the rotational speed of the disc, when returning to writing data into the main data region of the disc.
This has the disadvantage that it takes a noticeable time for the rotational speed of the disc to be changed to the required value, with the necessary degree of accuracy. As a result, writing data into the defect management area significantly increases the time taken to write the data. Similar problems arise when reading data which has been written into the defect management area.